Problem: Multiply.
Explanation: When a number is next to a variable, it means they are multiplied. So, $({7}{x^3})({3}{x^7})$ is the same as $({7})({x^3})({3})({x^7})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(7)(3)}{(x^3)(x^7)}$ Then simplify, and we're done! ${21}{x^{10}}$